1. Field of the Invention
The present invention relates to a method and system for producing cellulose acylate film. More particularly, the present invention relates to a method and system for producing cellulose acylate film in which unwanted precipitate as fine dust can be prevented and also productivity of solution casting can be kept high.
2. Description Related to the Prior Art
Polymer compounds are used to produce polymer film as optical film owing to advantageous characteristics. The polymer film is attached on a surface of a polarizing element as a constituent of a liquid crystal display panel (LCD). Examples of the optical films are a protection film of a polarizing element, and an optical compensation film (view angle enlarging film or the like), and anti-reflection film. Among various examples, cellulose acylate film as polymer film produced from cellulose acylate is advantageous for high optical property and easy handling in the production.
A solution casting process is a typical method of producing cellulose acylate film. In the solution casting, dope is cast on a casting support which is moving, the dope containing cellulose acylate, solvent and additives. Then a cast film of the dope is stripped from the support, to obtain polymer film by drying. According to this method, the cellulose acylate can be free from thermal damage. Polymer film with high transparency can be produced with advantage in the solution casting over another method of melt forming known in the field of polymer film.
However, fine dust occurs on a surface of a casting support such as a drum or a belt of metal with time after continuous use in the solution casting. This phenomenon is called plating out. In FIG. 4, dope 2 with various components is illustrated. Fatty acid ion (R—COO)− of a fatty acid R—COOH, and calcium ion Ca2+ of calcium Ca are initially contained in the cellulose acylate, and bind with one another to create insoluble precipitate of calcium salt (R—COO)2Ca of fatty acid. The fine dust is a result of presence of the insoluble precipitate on the surface. This is unfavorable in that the flatness of the cast film or cellulose acylate film will be lowered.
JP-A 2003-001654 discloses a method in which the film forming is stopped upon occurrence of plating out, and fine dust on the support is wiped by use of fabric with solvent. U.S.P. 2004/023051 (corresponding to JP-A 2004-107629) discloses a method in which stable fine particles as aid of filtration are disposed in the dope, the dope is passed through a filter sheet to form a porous layer on the filter sheet, so as to trap the insoluble precipitate. Occurrence of plating out is suppressed by casting dope after reduction of the insoluble precipitate.
However, JP-A 2003-001654 only discloses the method for use after occurrence of the plating out on the support. There is no prevention of the plating out. Also, the productivity may be low because the operation of film forming must be stopped. U.S.P. 2004/023051 (corresponding to JP-A 2004-107629) discloses the filter sheet having a small pore diameter for trapping fine particles of insoluble precipitate. Efficiency of filtration may be low because of the rise of the filtration pressure and filtration time.